Scalable production of structurally colored composite films by shearing supramolecular composites of polymers and colloids

Structurally colored composite films, composed of orderly arranged colloids in polymeric matrix, are emerging flexible optical materials, but their production is bottlenecked by time-consuming procedures and limited material choices. Here, we present a mild approach to producing large-scale structurally colored composite films by shearing supramolecular composites composed of polymers and colloids with supramolecular interactions. Leveraging dynamic connection and dissociation of supramolecular interactions, shearing force stretches the polymer chains and drags colloids to migrate directionally within the polymeric matrix with reduced viscous resistance. We show that meter-scale structurally colored composite films with iridescence color can be produced within several minutes at room temperature. Significantly, the tunability and diversity of supramolecular interactions allow this shearing approach extendable to various commonly-used polymers. This study overcomes the traditional material limitations of manufacturing structurally colored composite films by shearing method and opens an avenue for mildly producing ordered composites with commonly-available materials via supramolecular strategies.


Supplementary Methods
Synthesis of polystyrene (PS) colloids: PS colloids without carboxyl groups were also synthesized by emulsifier-free polymerization 1 .Briefly, 8.0 mL styrene and 400 mL water were mixed in a 500 mL flask.After de-oxygen, 0.28 g potassium persulfate was added to the above mixture, and the mixture was stirred for 6 h at 75 °C under N2 atmosphere.Finally, the PS colloids were separated by centrifugation and washed with ethanol and water three times.
First, 100 mL ethanol, 7 mL H2O, and 4 mL ammonium hydroxide were mixed.Then, 7 mL tetraethyl orthosilicate was added to the resulting mixture, and the mixture was stirred for 5 h at 30 °C.Then, 0.5 mL octadecyltrimethoxysilane was added, followed by a reaction at 40 °C for 12 h.Finally, the resultant products were thoroughly washed with ethanol three times by centrifugation.from SiO2-C18 and SiO2 colloids with PEI60k, respectively.Insets in (d and e): the corresponding 2D FFT images.
The dispersibility of colloids in polymer matrices was observed using SEM imaging technique.PS colloids without surface charges exhibited significant aggregation in the polymeric matrix (Supplementary Fig. 4b).After shearing treatment, no noticeable structural color was observed in the resulting composite film (Supplementary Fig. 4c insets).In contrast, SiO2-C18 and SiO2 colloids were uniformly dispersed in the polymeric matrix (Supplementary Fig. 4d, e), which could be attributed to the supramolecular interactions between the colloids and the polymer that promote the uniform dispersion of the colloids in the polymeric matrix.Upon shearing treatments, these composite films composed of SiO2-C18 and SiO2 colloids exhibited remarkable structural colors with reflection peak intensities of 40% and 55% (Supplementary Fig. 4c), respectively.These results revealed that the uniform dispersion of colloids in the polymeric matrix was a prerequisite for shear-induced ordering.Furthermore, we observed that appropriate supramolecular interactions between the polymer and the colloid significantly improved the shear-induced ordering effects.This can be explained by the fact that the appropriate supramolecular interactions might facilitate momentum transfer between the colloid and the polymer 4 , resulting in a well-ordered colloidal arrangement.composite film inside two PET sheets.c Reflection spectra of PEI60k-PS60 films under shear strains of 240% and 325% after 25 shearing passes (rod diameter: 3 mm, load: 5 N).Insets in (c): the corresponding photographs at viewing angles of 0° and 60° (sample diameter: 1.9 cm).d Reflection peak intensity of PEI60k-PS60 films at different shear strains as a function of the number of shearing passes.Error bars represent mean ± standard deviations.n = 3 independent experiments.e SEM images of PEI60k-PS60 films under shear strains of 240% (top panel) and 325% (down panel) after 25 shearing passes.

Supplementary Note 3:
To compare the ordering effects produced by shearing different supramolecular composites, the relevant processing parameters, including the thickness of the supramolecular composite (hComposite), the diameter of the rod (D), and the load (G), were optimized individually while other parameters were held constant.
In the bending-induced shearing treatment process, bending the sandwiched film around the rod generates a strong shearing force inside the composite film parallel to the surface 5 .The corresponding schematic representation is shown in Supplementary Fig. 6a.R is the radius of the rod, s is the unstretched length of the top and bottom PET films (green dotted lines), and  1 and  2 are the lengths of the top and bottom surfaces of the composite film in contact with two PET films of length s (purple solid lines), respectively.hComposite and hPET are the thicknesses of the composite film and PET, respectively.θ1 and θ2 are the angles at the center of the rod between the rod midpoint and the last contact points of the top and the bottom PET films of length s, respectively.
∆ is the displacement difference of the composite film between the top and the bottom PET planes during the shearing process.In this case, Therefore, the shear strain  can be calculated by Supplementary Equation 15 : To investigate the effect of shear strain (γ) on the shear-induced colloidal ordering, we prepared sandwiched films with thicknesses of 222 and 167 μm (Supplementary Fig. 6b), using a hot-press apparatus by adjusting the temperature and pressure during the process.The corresponding thicknesses of the resulting composite films were 108 and 53 μm.When the sandwich film was bent 180° around the rod, i.e.,  2 =  2 , the shear strains generated in the composite films with thicknesses of 108 and 53 μm can be calculated to be 240% and 325%, respectively, according to Supplementary Equation 1.For the same number of shearing passes (25 passes), we noted that the green appearance of the composite films did not show significant differences in brightness when the shear strain increased from 240% to 325%.The corresponding peak intensities increased by less than 10% (Supplementary Fig. 6c).Furthermore, the relationship between the peak intensity and the number of shearing passes showed no significant difference under shear strains of 240% and 325% (Supplementary Fig. 6d), which was further supported by observations from SEM images (Supplementary Fig. 6e).These results indicate that for current supramolecular composites, a shear strain of 240% induced by bending is sufficient to achieve shear-induced ordering.Notably, for larger shear strains, there were fewer stacked layers in the composite film, which would be detrimental to the overall reflectivity of the film.In this case, we employed a composite film with a thickness of ~ 100 µm for shearing treatment.

Supplementary Note 4:
The shear strain rate () during the shearing process can be calculated by Supplementary Equation 2.
Where  is the winding speed rate of the composite film during shearing treatment

Supplementary Note 5:
The rod diameter (D) significantly affects the compressive stress of the film, thereby affecting the shear-induced ordering effects of colloids 6 .To obtain high-quality structural colors, we further optimized the rod diameter.
Specifically, the sandwiched film was bent along rods with varying diameters under a fixed load of 2 N, while the reflective spectrometer recorded in situ the change in intensity of the reflection peaks at different shearing passes.Taking a PEI60k-PS60 film with a thickness of ~ 94.4 μm as an example (Supplementary Fig. 7a), when the rod diameter increased from 1.7 to 12 mm, the brightness of the green appearance of the PEI60k-PS60 film decreased significantly and gradually lost its angular dependence at the same number of shearing passes (Supplementary Fig. 7b).Correspondingly, the reflection peak intensity decreased from ~ 65% to ~ 0% (Supplementary Fig. 7c).These results imply that reducing the rod diameter promoted shear-induced colloidal ordering.
Notably, when the rod diameter was reduced from 3 to 1.7 mm, there was no significant change in the peak intensity of the corresponding composite film, indicating no significant improvement in its optical quality.This may be due to the fact that a smaller diameter rod generated a larger average compressive stress within the composite film at a given load, leading to more defects.The average compressive stress generated in the composite film on rods with different diameters during shearing treatment can be calculated using Supplementary Equation 3.These results suggested that rods with diameters of ~ 3-8 mm are suited for the shearing processing of current supramolecular composites (Supplementary Fig. 7d).
Supplementary Fig. 8. Illustration showing compressive stress generated by bending the sandwich film onto a rod under a specific load G.
As depicted in Supplementary Fig. 8, the average compressive stress  ̅ can be calculated by Supplementary Equation 3.
Where  is the weight of the load, and  ′ is the projected area of the contact surface between the rod and the sandwich film along the z-direction (i.e., thickness direction, depicted in Supplementary Fig. 6a).w is the width of the sandwiched film along the ydirection (30 mm).Therefore, for a composite film sheared under different rods with diameters of 1.7, 3, 5, 8, 10, and 12 mm,  ̅ can be calculated as 39, 22, 13, 8.3, 6.7, and 5.5 kPa.
stresses were calculated to be 0.29, 0.71, and 1.43 MPa, respectively, according to Supplementary Equation 4. Other processing parameters were fixed, including a rod diameter of ~ 3 mm, a composite film thickness of ~ 100 μm, and a PET thickness of ~ 57 μm.As shown in Supplementary Fig. 9a, compared to the load of 2 and 5 N, the PEI60k-PS60 film showed a higher reflection peak intensity under the high load of 10 N during shearing.This can be attributed to high shear stress induced by the high load, which promoted polymer chain stretching and colloidal momentum transfer 4,8 .In addition, by quantitatively studying the variation in the reflection peak intensity versus the number of shearing passes under different loads, we found that 80 shearing passes were required to construct structurally colored composite films (SCCFs) with a reflection peak intensity of up to 55% at a load of 2 N.In comparison, only 42 shearing passes were required at a load of 5 N, and 32 shearing passes were needed for a load of 10 N (Supplementary Fig. 9b, c, and d).These results indicate that the magnitude of the load is critical for obtaining SCCFs with high-quality structural colors.It is worth noting that when the load reached 10 N, defects appeared in the composite films during shearing treatment (Supplementary Fig. 9d), which may be related to the mechanical strength of the composite.Therefore, a load of 5 N is optimal for constructing composite films with high-quality structural colors.with the carboxyl groups on the surface of PS-COOH 10 (Supplementary Fig. 12a).It is known that linear polyether chains become entangled at a critical Mn of ~ 6-7 kDa 11 .

Viscoelastic properties of neat
Therefore, neat PEG-b-PPG-b-PEG with Mn of 2.9 kDa, 8.4 kDa, and 14.6 kDa exhibited viscous liquid behavior with nonentanglement, slight entanglement, and significant entanglement, respectively.These PEG-b-PPG-b-PEG polymers were chosen to construct supramolecular composites, and we used photography, reflection spectroscopy, and SEM imaging to investigate shear-induced ordering effects.After shearing treatment, the composite consisting of PEG-b-PPG-b-PEG with Mn of 2.9 kDa showed no obvious change in appearance and colloidal structure compared to that before shearing, with angle-independent structural colors and low-ordered colloidal structures (Supplementary Fig. 12b, c).In contrast, the composite consisting of PEGb-PPG-b-PEG with Mn of 8.4 kDa showed a weak and low angle-dependent structural color (Supplementary Fig. 12d), while the composite consisting of PEG-b-PPG-b-PEG with Mn of 14.6 kDa showed a strong angle-dependent structural color and a highly ordered colloidal structure (Supplementary Fig. 12e, f).These results suggested that chain entanglement of polymers in the supramolecular composite can promote the colloid to obtain sufficient momentum to form ordered colloidal arrangements.This result is consistent with PEIs and implies the universality of the shear-induced ordering mechanism associated with Mn.
(Supplementary Fig. 22d).The productivity at 60 °C was 13 times higher than that at 30 °C for the same shear condition.The significant reduction in the number of shearing passes and the increased productivity by the slight increase in processing temperature demonstrates the unique processing advantages of the supramolecular composite for the construction of high-quality SCCFs.

( 1 .Supplementary Fig. 7 .
8 m• min -1 ), therefore, for a composite film with a thickness of 108 μm, ̇ can be calculated as 278 s -1 .Optimization of the diameter of the rod.a Optical microscopy image of the cross section of the sandwiched film, including two PET films and a composite film.b, c Photographs and reflection spectra of PEI60k-PS60 films produced under 100 shearing passes with a fixed load (G: 2 N) and the thickness of the PEI60k-PS60 film (hComposite: ~ 94.4 μm) but varying rods with diameters of 1.7, 3, 5, 8, 10, and 12 mm.d Reflection peak intensity of PEI60k-PS60 films produced by using rods with different diameters as a function of the number of shearing passes.Error bars represent mean ± standard deviations.n = 3 independent experiments.

PEIs with different molecular weights and corresponding composites Supplementary Fig. 10 .Supplementary Note 7 :
The viscosity of PEI600, PEI10k, and PEI60k as a function of angular frequency (ω).The measurement was performed with a strain amplitude of 0.1%.composite films consisting of PEG-b-PPG-b-PEG with Mn of 8.6 kDa and 14.6 kDa after shearing treatments.Insets in (d and e): the corresponding photographs at viewing angles of 0° (top) and 60° (bottom) (sample diameter: 2.4 cm).f SEM images of the composite film composed of PEG-b-PPG-b-PEG with Mn of 14.6 kDa before and after shearing treatments.To investigate the universality of the shear-induced ordering mechanism associated with Mn, PEG-b-PPG-b-PEG with different Mn and PS-COOH was chosen because the ether groups on the polymer chains can form hydrogen bonds

Table 1 .
Refractive indices of the matrix polymer and colloid of